Marine transport of international goods transport has become widely accepted due to large volume, security, economy and other advantages, nearly 90% of global trade is transported by ocean. Followed by a sharp increase in the number of ocean vessel, and thus caused serious air pollution and ocean pollution. The international requirements on emissions of SOx are determined by the MARPOL 73/78 Annex VI, which specifies a global limit and a local limit on the sulphur content in marine fuel. The specified sulphur limit will change according to the illustration in Fig. below, showing a reduction in ECA from 1.0% to 0.1% sulphur in 2015, and a reduction in the global limit from 3.5% to 0.5% sulphur in 2020.

When open-loop mode is used, the natural chemical composition of seawater neutralizes the impact of SOx in the scrubber water. The water is taken directly from the sea and fed to the scrubber. The water is discharged into the sea without any further treatment. When closed-loop mode is used, the seawater is circulated to spray, to neutralise the sulphuric acid in the wash water, an addition of chemicals (NaOH) is needed. To avoid an increase in salinity and contamination of the system, the sulphate and PM must be removed continuously, and add some seawater. Before discharging the wash water, the cleaning process is performed in a water cleaning unit (WCU) and the sludge is led to a sludge tank to meet the IMO criteria. The hybrid desulfurization system combines open-loop and closed-loop processes, and has two running modes, which provide maximum flexibility. The open-loop mode is typically used in open waters where the alkalinity is sufficiently high for effective scrubbing. The closed-loop mode is used in certain enclosed waters, harbours and estuaries or where the alkalinity of the seawater is low. This combination optimises the chemical consumption and ensures that discharges do not affect sensitive areas.